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SEYMOUR  DURST 


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[Prom  the  American  Journal  of  Science,  Vol.  XXXIX,  May,  1890.] 


ON  THE 

METAMORPHIC  STRATA  OF  SOUTHEASTERN 
NEW  YORK. 

By  Frederick  J.  H.  Merrill. 


Of  the  strata  comprised  in  the  metamorphic  terrane  of 
southeastern  New  York  there  are  two  principal  divisions. 

First,  in  Rockland,  Orange,  Putnam  and  Dutchess  counties, 
the  granitoid  gneisses  and  granulites  of  the  Highlands,  which, 
with  their  continuation  in  New  Jersey,  can  be  shown  strati- 
graphically  to  be  pre-Cambrian,  since  they  underlie  unconform- 
able in  southern  Dutchess  County,  N.  Y.,  and  at  several  points 
in  New  Jersey,  a  basal  Paleozoic  quartzite  of  Potsdam  or 
Lower  Cambrian  age. 

Secondly,  in  Westchester  and  New  York  Counties  the  gneis- 
soid  quartzites  and  arkoses  overlain  by  crystalline  limestones  and 
mica  schists,  which  border  the  rocks  of  the  first  division  on  the 
southeast  and  have  been  regarded  as  altered  Paleozoic  rocks  of 
Ordovician  or  Cambrian  age  by  Professors  W.  W.  Mather  and 
J.  D.  Dana. 

The  metamorphic  rocks  of  the  New  Jersey  Highlands  were 
first  definitely  classified  by  Dr.  N.  L.  Britton  as  a  result  of  his 
studies  in  1885  and  1886.*  In  this  terrane  he  identified  three 
groups  :  first ;  a  Massive  Group,  devoid  of  bedding  planes, 

*  Annual  Report  G-eol.  Survey  of  New  Jersey,  1885-1886. 


384:  Merrill — Metamorphic  Strata  of  8.  K  New  York. 


described  by  him  as  being  the  oldest  formation  exposed  in  that 
state,  and  almost  entirely  composed  of  "  hornblendic  granite"  : 
second,  an  Iron-bearing  Group,  resting  conformably  on  the  pre- 
ceding, well  stratified,  and  comprising  bedded  granulites  and 
magnetite  deposits,  above  the  latter  being  crystalline  limestones 
containing  magnesian  silicates  in  various  stages  of  alteration  : 
third  and  uppermost,  a  "  Schistose  Group,"  composed  of  mica- 
ceous gneisses,  of  mica  schists  sometimes  containing  graphite, 
and  hornblendic  and  pyroxenic  schists  of  varied  composition. 
The  basal  Paleozoic  quartzite  has  not  yet  been  found  to  rest 
on  strata  of  the  third  group,  and  the  relative  age  of  the  latter 
remains  in  doubt. 

In  the  study  of  the  metamorphic  rocks  of  the  New  Jersey 
Highlands  and  in  a  preliminary  examination  of  the  Hudson 
River  section,  the  writer  was  associated  with  Dr.  Britton  and 
has  since  made  a  more  extended  study  of  the  latter.  The  facts 
there  observed  corroborate  the  general  results  of  Dr.  Britton's 
stratigraphical  work  in  New  Jersey. 

The  basal  member  of  the  pre-Cambrian  terrane  of  southeast- 
ern New  York  and  New  Jersey,  is  a  granitoid  hornblende- 
gneiss  consisting  chiefly  of  quartz,  orthoclase,  plagioclase  and 
hornblende.  Magnetite  and  zircon  sometimes  occur  as  acces- 
sories. As  stated  by  Dr.  Britton  the  rock  is  devoid  of  bedding 
planes  and  has  only  a  parallel  arrangement  of  its  minerals  to 
denote  macroscopically  its  sedimentary  origin.  Its  microscopic 
structure,  however,  also  suggests  its  detrital  and  metamorphic 
character. 

This  rock  is  not  a  granite  although  usually  called  by  that 
name.  In  addition  to  the  parallel  arrangement  of  its  minerals 
which  would  differentiate  it  from  the  normal  granites,  its  struc- 
ture shows  that  it  never  attained  that  condition  of  molecular 
freedom  which  would  be  necessary  to  enable  its  mineral  com- 
ponents to  assume  a  crystalline  form.  The  quartzes  and  feld- 
spars occur  in  irregular  masses  interlocking  at  their  margins 
and  in  no  case  show  crystalline  boundaries.  The  hornblende, 
which  is  allotriomorphie  and  tills  the  irregular  interstices  be- 
tween the  other  mineral  particles,  has  probably  been  developed 
in  the  process  of  metamorphism  from  aluminous  mud  contain- 
ing iron,  lime  and  magnesia.  As  in  rocks  which  have  been 
developed  from  the  plastic  or  fluid  state  the  magnesia-iron 
silicates  according  to  Rosenbusch,  are  idiomorphic  with  respect 
to  the  feldspars  and  the  latter  are  idiom orphic  with  respect  to 
the  quartzes,  the  structure  of  the  rock  in  question  seems  to 
demonstrate  that  its  component  minerals  are  not  developed 
from  a  magma  but  are  merely  the  fragmental  particles  of  a  sedi- 
mentary rock  and  that  during  metamorphism  only  a  slight  de- 
gree of  molecular  freedom  was  attained  by  them. 


Merrill — Metamorphic  Strata  of  S.  K  New  York.  385 


The  thickness  of  the  basal  member  is  at  present  indeter- 
minate, as  nothing  has  been  found  beneath  it,  and  in  the  Hud- 
son River  section  it  is  seen  to  descend  below  tide-level.  The 
maximum  thickness  exposed  is  in  Breakneck  Mountain,  of 
which  the  summit  is  said  to  be  1787  feet  above  tide.  What- 
ever be  the  true  height  of  the  mountain,  its  central  mass  from 
tide-level  to  the  summit  consists  of  hornblende-gneiss,  and 
whether  it  be  anticlinal  or  monoclinal  in  structure,  an  equiv- 
alent thickness  of  this  rock  is  exposed. 

The  second  member  or  Iron-bearing  Group,  as  Dr.  Britton 
has  stated,  consists  essentially  of  granulites,  this  name  being 
used  by  him  and  the  present  writer  to  describe  subcrystalline, 
well  stratified  metamorphic  rocks  composed  principally  of  de- 
trital  quartz  and  feldspar.  They  are  highly  metamorphosed 
arkoses,  and  differ  essentially  from  the  under-lying  formation 
in  the  absence  of  magnesia-iron  silicates  such  as  hornblende  and 
biotite.  The  comparative  absence  of  silicate  minerals  contain- 
ing iron  is  indeed  the  conspicuous  feature  of  this  member  taken 
as  a  whole.  In  some  places,  as  at  Garrisons,  a  pale  yellowish 
mica  is  present,  giving  the  rock  the  semblance  of  a  granite, 
under  which  name  it  is  quarried,  and  in  close  proximity  to  some 
of  the  magnetite  beds  in  New  Jersey,  biotite  and  hornblende 
are  quite  abundant.    This  formation  is  at  least  500  feet  thick. 

The  beds  of  magnetite  appear  to  occur  at  various  levels  in 
the  second  group  and  offer  little  evidence  of  their  origin.  If, 
however,  it  were  known  that  the  rock  which  furnished  the  de- 
tritus out  of  which  these  granulites  were  formed,  contained 
magnetite  distributed  through  it  in  any  appreciable  quantity 
there  would  be  good  reason  for  the  conclusion  that  the  magne- 
tite beds  of  this  horizon  originated  as  beds  of  magnetic  sand 
concentrated  by  wave  action.  There  is  no  indication  that  they 
originated  as  bog  ores.  The}r  are  simply  lenticular  beds  en- 
closed by  a  stratified  metamorphic  rock  formed  of  quartz  and 
feldspar  sand  and  there  is  no  adjoining  rock  of  which  the  com- 
position would  suggest  that  it  is  a  metamorphosed  soil.  There 
is,  besides,  no  indication  of  a  former  land  surface  on  which 
these  beds  might  have  been  formed.  The  only  suggestion  of 
organic  agency  is  in  the  apatite  which  occurs  with  the  iron  ores 
at  some  of  the  mines  and  this,  if  it  were  organic,  could  have 
been  derived  from  marine  organisms  as  well  as  from  terres- 
trial. It  is  not,  however,  by  any  means  certain  that  this  apatite 
is  organic,  since  the  mineral  is  a  frequent  accessory  in  various 
eruptive  rocks. 

The  strata  of  Dr.  Britton's  schistose  group  and  the  lime- 
stones which,  according  to  him,  occur  near  the  top  of  the  sec- 
ond member  are  found  near  the  Hudson  River,  in  Orange 
County  at  Fort  Montgomery,  and  in  Putnam  County  between 


386    Merrill*  Metamorphic  Strata  of  8.  E.  New  York. 


Highlands  and  Garrisons.  It  has  not  yet  been  determined 
whether  they  should  be  regarded  as  part  of  the  system  to  which 
the  two  lowest  groups  or  members  belong,  nor  have  their  rela- 
tions been  determined  with  respect  to  the  Manhattan  Group 
(hereafter  defined).  As  before  stated,  no  Ordovician  or  Cam- 
brian Rocks  have  been  found  to  rest  upon  them  and  hence 
their  pre-Cambrian  age  cannot,  as  yet,  be  predicated  with  cer- 
tainty. 

The  thickness  of  the  pre-Cambrian  rocks  in  the  Hudson  River 
Valley  may,  therefore,  be  stated  as  between  2300  and  2800 
feet.  As  to  their  age,  it  is  difficult  to  predicate  anything  on 
account  of  their  isolated  position.  The  "  upper  member,"  or 
granulitic  group  may  or  may  not  be  the-  equivalent  of  the 
Huronian.  There  is  nothing  but  its  unconformable  position 
immediately  below  the  Cambrian  quartzite  to  suggest  such 
equivalence,  and  as  it  seems  to  be  conformable  to  the  "  basal 
member"  that  would  have  to  be  included  in  the  same  horizon. 

The  magnetite  beds  of  this  region  have  been  compared  by 
Dr.  Britton  to  those  of  the  Grenville  series  in  Canada,  and 
they  may  be  equivalent  but  until  the  Laurentian  of  Eastern 
Canada  has  been  studied  and  classified  with  the  aid  of  modern 
methods  of  research  any  attempt  at  correlation  will  be  unsatis- 
factory. 

The  stratigraphy  of  the  Highland  region  as  displayed  in  the 
section  along  the  Hudson  River  is  very  simple.  A  small  num- 
ber of  anticlinal  ridges,  900  to  1700  feet  in  height,  with  a 
northeasterly  trend,  are  intersected  by  the  Hudson  River  valley. 
Along  the  lines  of  these  the  basal  member  of  the  Archaean  is 
exposed  and  resting  on  their  flanks  and  in  the  synclinal  troughs 
the  rocks  of  the  Iron-bearing  Group  appear. 

The  most  northern  axis  is  that  of  the  Fishkill  range  which  is 
probably  a  continuation  of  the  Wawayanda  Mountain  axis  of 
Northern  New  Jersey,  along  which  line  in  Orange  Co.  N.  Y. 
occur  a  number  of  isolated  hills  of  gneissic  rock  known  as 
Sugar  Loaf  Mountain,  Goose-Pond  Mountain,  Peddler  Hill, 
Rainer  Hill,  Mosquito  Hill,  Round  Hill  and  Woodcock  Hill. 

Second  is  that  of  Storm  King  and  Breakneck  Mountains, 
closely  related  orographically  to  the  axis  of  Crow  Nest  Moun- 
tain and  Bull  Hill  or,  as  the  latter  is  sometimes  called,  Mt. 
Taurus.  There  is  probably  a  fault  line  between  these  two  axes 
and  nearly  parallel  to  them,  but  the  structural  details  have  not 
been  accurately  determined.  A  fourth  axis  is  that  which 
crosses  the  Hudson  at  AVest  Point.  A  group  of  axes  crosses 
the  Hudson  along  the  lines  of  Fort  Hill,  Sugar  Loaf  Moun- 
tain, Anthony's  Nose  and  Bear  Hill  and  finally  the  anticlinal 
of  Manitou  Mountain  and  the  Dunderberg  closes  the  succession. 
These  folds  generally  pitch  steeply  to  the  S.W.     In  this  re- 


Merrill — Metamorphic  Strata  of  8.  E.  New  York.  387 


spect  the  most  noticeable  are  those  of  Anthony's  Nose  and 
Sugar  Loaf,  the  latter  of  which  exhibits  this  peculiarity  by  its 
profile  to  the  most  careless  observer. 

On  the  southeastern  flank  of  Dunderberg  and  Manitou 
Mountains  the  stratified  granulites  again  appear  and  these  are 
succeeded  after  a  drift-covered  interval  near  Peekskill  and 
Jones  Pt.  by  slightly  metamorphosed  sandstones,  limestones 
and  slate,  presumably  of  Paleozoic  age,  the  relation  of  which 
to  the  sub-crystalline  rocks  of  Westchester  and  New  York 
Counties  is  still  an  interesting  problem. 

Younger  Hocks. — The  metamorphic  strata  of  New  York 
and  Westchester  Counties  have  long  attracted  the  attention  of 
geologists  and  several  attempts  have  been  made  to  solve  the 
problems  of  their  age  and  history.  The  extended  researches 
of  Prof.  Jas.  D.  Dana  on  the  relations  of  the  limestone  belts 
in  the  vicinity  of  Manhattan  Island  have  furnished  a  clue  to 
the  stratigraphy,  and,  after  a  careful  study  of  a  portion  of  the 
terrane  to  which  those  limestones  belong,  the  writer  is  enabled 
to  announce  the  following  general  results.  As  Prof.  Dana  has 
noted  (Am.  Jour.  Sci.,  Ill,  vol.  xxi,  p.  439),  the  beds  underlying 
the  limestones  of  New  York  County  are  highly  quartzose, 
while  those  overlying  them  are  chiefly  micaceous.  Throughout 
Westchester  County  south  of  the  latitude  of  Sing  Sing,  an 
area  of  about  one  hundred  square  miles,  the  writer  has  found 
this  lithological  difference  to  prevail.  The  exact  relation  of 
the  lower  beds  to  the  granitoid  gneisses  and  granulites  of  the 
Highlands  of  Rockland,  Orange  and  Putnam  Counties  has  not 
yet  been  determined  by  the  writer,  but  his  investigations  have 
satisfied  him  that  the  former,  with  the  exception  of  the  lime- 
stones, are  distinctly  detrital  rocks  in  which  are  preserved  the 
fragmental  character  of  the  quartz  and  feldspar  which  they 
contain.  The  mica,  chiefly  biotite,  is  of  metamorphic  origin, 
having  been  developed  from  aluminous  mud  rich  in  potash, 
iron  and  magnesia. 

As  the  limestones  of  the  region  under  consideration  contain 
no  organic  traces,  so  far  as  we  know,  there  is  no  direct  clue  to 
their  origin,  but  in  the  absence  of  evidence  to  the  contrary  we 
may  believe  that  the  carbonate  of  lime  was  separated  by  organic 
agencies  from  the  sea  water  which  held  it  in  solution.  The 
well  known  chemical  theory  of  Dr.  Hunt  accounts  very  satis- 
factorily for  the  presence  of  bicarbonate  of  lime  in  sea  water, 
but  it  does  not  account  so  well  for  the  separation  of  the  car- 
bonate from  solution,  and  as  this  separation  is  effected  very 
extensively  at  the  present  time  by  various  marine  organisms 
and  as  the  cases  of  chemical  separation  of  carbonate  of  lime 
from  solution  in  sea  water  are  very  few,  we  are  justified  in  be- 
lieving that  the  former  process  was  in  operation  as  far  back  in 


388  Merrill — Metamorphic  Strata  of  S.  F.  New  York. 


the  earth's  history  as  the  sea  water  was  inhabited  by  creatures 
possessing  skeletons  of  carbonate  of  lime.  The  source  of  the 
magnesia  so  abundant  in  the  limestone  is  yet  undetermined. 

The  lowest  stratum  yet  recognized  among  the  Westchester 
County  rocks  is  a  reddish  gneiss  which  forms  the  central  mass 
of  some  of  the  hills  in  Yonkers  where  it  is  well  exposed,  and 
consists  of  small  grains  of  detrital  quartz  with  fragments  of 
reddish  orthoclase  and  a  few  crystals  of  biotite  which  have 
developed  during  the  process  of  metamorphism.  From  its 
macroscopical  characters  the  rock  would  be  called  a  gneiss. 
From  its  microscopic  structure  it  would  be  called  a  metamor- 
phosed sandstone  or  arkose.  As  the  name  quartzite  gneiss  has 
been  applied  in  Germany  to  rocks  of  analogous  structure,  it  is 
proposed  to  designate  the  rock  in  question  as  an  arkose  gneiss. 
It  may  be  objected  that  all  gneisses  were  quartzites  or  arkoses 
previous  to  their  metamorphism,  but  there  is  an  evident  neces- 
sity for  the  use  of  some  descriptive  term  which  will  convey 
the  idea  of  comparatively  unaltered  detrital  structure  and 
differentiate  such  a  rock  as  this  from  the  pre-Cambrian  gran- 
itoid gneisses  of  Putnam  County. 

The  thickness  of  the  red  gneiss  referred  to  has  not  been 
determined,  as  no  section  has  been  found  which  shows  its  rela- 
tions to  the  formation  beneath  it,  but  it  is  believed  to  rest 
upon  the  stratified  granulites  which  form  the  second  or  upper 
member  of  the  pre-Cambrian  formation.  Since  the  best  ex- 
posures of  the  red  gneiss  have  been  found  within  the  limits  of 
Yonkers  and  its  relations  to  the  overlying  stratum  are  well 
shown  within  the  limits  of  that  city,  it  is  proposed  to  call  it 
the  Yonkers  gneiss.  Outcrops  of  this  rock  are  frequent  along 
the  shores  of  the  Hudson  and  good  exposures  may  be  found  at 
Hastings,  on  the  property  of  Dr.  Draper ;  on  the  river  near 
the  southern  border  of  Tarrytown  ;  between  Scarborough  and 
Sing  Sing  stations  near  the  railroad  ;  along  the  southeast  shore 
of  the  mouth  of  Croton  Bay  on  the  property  of  Orlando  B. 
Potter,  Esq. ;  and  a  little  south  of  Oscawana  Station,  on  Osca- 
wana  Island :  it  also  probably  occurs  in  the  ridge  which  sepa- 
rates Annsville  Cove  from  the  valley  of  Peekskill  village. 
The  best  exposures  are  in  Yonkers,  on  and  near  Jerome 
avenue,  a  little  north  of  the  Xew  York  City  line. 

With  regard  to  the  variations  in  the  composition  and  struc- 
ture of  the  Yonkers  gneiss,  the  most  important  are  an  increase 
in  the  proportion  and  size  of  the  feldspar  fragments  in  ap- 
proaching the  Lauren tian  Highlands. 

Overlying  the  reddish  Yonkers  gneiss  and  beneath  the  crys- 
talline limestone  is  a  stratum  of  thinly  bedded  gray  quartzite 
gneiss.  This  contains  but  little  feldspar  and  its  component 
beds  vary  in  composition  from  almost  pure  quartz  to  a  mixture 


Merrill — Metamorpkic  Strata  of  S.  K  JVeiv  York  389 

of  quartz  and  biotite  or  hornblende.  Occasionally,  layers  of 
pure  biotite  schist,  an  inch  or  two  in  thickness,  are  intercalated 
with  white,  coarsely  granular  quartzite.  This  rock  forms  the 
summit  and  eastern  slope  of  the  ridge  which  separates  the 
Sawmill  River  valley  from  that  of  the  Hudson  and  also  occurs 
in  that  on  the  east  side  of  the  Hudson  between  Yonkers  and 
Spuyten  Duyvil.  It  forms,  as  well,  the  anticlinal  ridge  of 
Fordham  Heights  which  borders  the  east  shore  of  the  Harlem 
River  and  of  which  the  southern  extremity  forms  the  long 
narrow  hill  by  the  northern  end  of  Seventh  avenue  and  which 
separates  the  latter  from  the  Boulevard. 

In  general  terms  this  quartzite  gneiss  is  the  gray  rock  used 
for  a  building  stone  in  southern  Westchester  County.  The 
Yonkers  gneiss  is  also  used  for  building,  but  not  so  extensively 
and  is  characterized  by  its  reddish  color.  ISo  section  has  yet 
been  found  which  shows  accurately  the  thickness  of  this  gray 
gneiss.  It  is  at  least  two  hundred  feet  thick  and,  in  many  ex- 
posures, has  an  aggregate  thickness  of  half  a  mile,  but  in  these 
it  is  apparently  repeated  many  times  by  folding.  As  in  most 
cases  the  folds  are  isoclinal  and  their  arches  have  been  removed 
by  erosion,  there  is  little  stratigraphical  evidence  of  the  thick- 
ness of  the  stratum  thus  folded. 

Since  this  rock  is  well  exposed  and  its  stratigraphical  rela- 
tions are  well  shown  in  the  Fordham  Heights  it  is  proposed  to 
call  it  the  Fordham  gneiss. 

The  Fordham  gneiss  varies  widely  from  the  normal  type,  in 
places,  through  the  presence  of  hornblende  and  garnet  and  an 
increase  in  the  amount  of  feldspar  and  mica,  but  the  localities 
of  variation  are  comparatively  few. 

Between  the  Fordham  gneiss  and  the  crystalline  limestone 
of  the  Hastings  Quarry  and  in  Yonkers  at  the  south  end  of 
the  railroad  trestle  near  the  Lowerre  Race  Track,  a  stratum  of 
thinly  bedded  quartzite  from  five  to  ten  feet  thick  is  found. 
It  seems  probable  that  this  is  of  wide  distribution  but  on  ac- 
count of  its  slight  thickness  it  is  very  rarely  preserved  when 
the  limestone  which  rested  upon  it  has  been  removed  by 
erosion. 

At  Tuckahoe  this  quartzose  stratum  next  beneath  the  lime- 
stone contains  numerous  flakes  of  biotite. 

The  position  and  stratigraphy  of  the  limestone  areas  of 
Westchester  County  have  been  carefully  studied  by  Professor 
Dana.  My  only  contribution  to  the  geology  of  these  beds  has 
been  to  determine  the  relations  and  character  of  the  associated 
rocks,  to  note  a  few  unrecorded  outcrops  and  in  a  few  cases  to 
extend  the  limits  of  those  previously  known. 

Professor  Dana  has  estimated  the  thickness  of  the  bed  oc- 
curring in  Tremont  and  the  Harlem  River  valley  at  from  six 


390    Merrill— Metamorpkie  Strata  of  &  K  New  York. 


hundred  to  seven  hundred  and  fifty  feet.  The  measurements 
of  the  writer  would  indicate  that  the  thickness  varies  from  six 
hundred  to  eight  hundred  feet,  it  being  apparently  greater  on 
New  York  Island  than  in  Morrisania.  The  eastern  bed  at 
Tuckahoe  is  but  one  hundred  and  fifty  feet  thick.  For  this 
rock  I  propose  the  name  of  Inwood  limestone,  from  the  locality 
on  New  York  Island  in  the  vicinity  of  which  it  is  well  exposed. 

The  rocks  which  overlie  the  limestone  are  highly  schistose 
and  consist  largely  of  mica  with  a  small  proportion  of  quartz 
and  usually  little  or  no  feldspar.  Garnet,  staurolite,  fibrolite 
and  cyanite  are  the  chief  accessories.  There  are  some  beds  of 
gneiss  among  them,  but  these  are  very  small  and  the  studies  of 
the  writer  enable  him  to  state  positively  that  mica  preponder- 
ates in  the  rocks  above  the  limestone  beds.  Xo  sections  have 
yet  been  found  which  would  warrant  an  expression  of  opinion 
as  to  the  exact  thickness  of  these  schists,  but  it  probably  ex- 
ceeds one  thousand  feet. 

The  mica  schist  formation  which  belongs  above  the  lime- 
stone is  of  very  limited  extent  in  Westchester  County.  A 
synclinal  ridge  of  this  rock  extends  from  Park  Hill,  in  Yon- 
kers,  northward  along  the  east  bank  of  the  Saw  Mill  River 
and  has  been  traced  to  Elmsford. 

North  of  Croton  landing  the  mica  schist  containing  garnet 
and  staurolite  extends  along  the  bank  of  the  Hudson  for  about 
a  mile  and  east  to  the  norite  area  of  the  Cortlandt  Series. 
Xear  Crugers  the  schists  have  been  described  by  Professor 
Dana 

Between  the  Bronx  River  and  Long  Island  Sound,  in  South- 
ern Westchester  County,  there  is  a  considerable  extent  of 
mica  schist  but  its  limits  are  not  determined.  In  Eastchester 
village,  on  the  west  shore  of  Eastchester  Creek  the  rock  is 
a  gneissoid  quartzite.  At  New  Rochelle  the  rock  along  the 
shore  of  the  Sound  probably  belongs  below  the  limestone.. 
The  same  rock,  essentially  a  gneissoid  quartzite,  occurs  on  the 
shore  of  Mamaroneck  Harbor,  while  Milton  Point  in  Rye 
township  seems  to  be  composed  of  the  mica  schists.  On  the 
Hudson  River  shore,  in  general,  the  limestone  areas  are  suc- 
ceeded to  the  north  by  mica  schists  and  to  the  south  by  the 
arkose  gneisses. 

As  these  uppermost  beds  are  well  exposed  on  Manhattan 
Island  of  which  they  constitute  the  principal  rock  formation 
they  may  well  be  called  the  Manhattan  schists. 

The  name  Manhattan  Group  was  proposed  in  1868  by  R.  P. 
Stevens,  Esq.,  to  include  the  rocks  of  New  York  Island  and  it 
seems  proper  that  it  should,  for  the  present,  be  retained,  in- 
cluding in  it.  with  the  Manhattan  schists,  the  Inwood  limestone 
and  the  Fordham  gneiss,  the  Yonkers  gneiss  which  though  not 


Merrill — Metamorphic  Strata  of 8.  E.  New  York.  391 


found  on  Manhattan  Island,  is  evidently  a  part  of  the  same 
littoral  deposit  to  which  the  Fordham  gneiss  belongs.  The 
Manhattan  schists  are  the  only  beds  now  to  be  found  on  New 
York  Island  with  the  exception  of  the  limestone  areas  de- 
scribed and  mapped  by  Professor  Dana  and  the  small  area  of 
Fordham  gneiss  at  the  north  end  of  Seventh  avenue. 

If  in  time  this  group  be  correlated  with  some  other  which 
has  been  previously  described,  the  names  here  suggested  may 
be  unnecessary,  but  until  the  question  is  indisputably  settled 
they  are  of  use  in  referring  to  the  formation  and  its  sub- 
divisions. 

Intercalated  with  the  Manhattan  schists  and  also  with  the 
beds  of  the  Fordham  gneiss  we  find  at  a  great  number  of 
localities,  hornblendic  and  augitic  strata  of  limited  thickness, 
usually  only  a  few  feet.  In  composition  these  rocks  resemble 
diorites  and  diabases,  and  in  structure  they  are  granular,  but 
their  present  well  stratified  condition  renders  it  difficult  to  say 
whether  they  are  originally  eruptive  rocks  or  not.  Whatever 
their  origin  they  are  now  metamorphic  rocks  and  as  such  may 
be  called  amphibolites  and  pyroxenites  according  to  the  ter- 
minology of  Kalkowsky.  It  is  probable  that  to  rocks  of  this 
character  we  are  indebted  for  some  of  our  serpentines,  notably 
that  of  60th  street  near  10th  avenue,  New  York  City,  for  as 
originally  suggested  by  Dana  and  lately  demonstrated  by 
Gratacap  it  is  derived  from  the  alteration  of  amphibole,  and 
on  61st  street  near  11th  avenue,  a  bed  of  amphibolite  occurs, 
of  which  the  line  of  strike  passes  through  the  well  known  and 
interesting  serpentine  above  mentioned  and  which  lies  about 
250  feet  southeast. 

Age  of  the  Manhattan  Group. — It  is  not  yet  in  the  power 
of  the  writer  to  contribute  any  positive  information  on  this 
important  question  He  has  not  yet  found  any  decisive  evi 
dence  of  the  age  of  the  rocks  in  question.  All  the  suggestive 
evidence,  however,  favors  the  view  taken  by  Professor  W.  W. 
Mather  and  subsequently  elaborated  by  Professor  J.  D.  Dana, 
viz  :  that  the  rocks  of  the  Manhattan  Group  are  the  metamor- 
phosed equivalents  of  the  Paleozoic  beds  of  Southern  Dutchess 
County. 

After  a  careful  study  of  the  stratigraphy  in  the  vicinity  of 
Peekskill  which  seems  to  be  the  index  of  this  geological 
chapter,  and  at  other  points  along  the  northern  margin  of  the 
Manhattan  terrane,  the  writer  concludes  that  if  this  group  is 
pre-Cambrihn,  its  identity  as  such  has  been  obscured  by  a  series 
of  stratigraphic  vicissitudes  so  complicated  that  it  is  beyond  his 
powers,  at  present,  to  conceive  them. 


392    Mrr  rill — Metamorphu:  St  nit  a  of 8.  K  New  York. 


A  fact  which  may  be  of  much  significance,  is  that,  the  Pots- 
dam or  Lower  Cambrian  sandstone  of  Southern  Dutchess 
County  lies  unconformably  on  the  second  or  granulitic  member 
of  the  pre-Cambrian  formation,  and  it  is  upon  the  same  second 
member  that  the  basal  beds  of  the  Manhattan  Group  rest. 
No  unconformability  has  yet  been  found  between  the  Manhat  • 
tan  Group  and  the  underlying  pre-Cambrian  beds,  and  it  is 
chiefly  this  lack  of  positive  evidence  that  leaves  the  writer  in 
doubt  as  to  the  geological  equivalence  of  the  former.  Of 
equal  significance,  however,  is  the  lack  of  unconformability 
between  the  Lower  Silurian  strata  of  Peekskill  Hollow,  Tomp- 
kins Cove,  and  Verplank's  Point,  which  are  but  partially 
metamorphosed,  and  the  metamorphic  beds  of  the  Manhattan 
Group  which  adjoins  them. 

The  crystalline  limestones  of  the  Manhattan  Group  are,  as 
already  stated,  highly  magnesian  and  in  this  respect  they  cor- 
respond in  composition  to  the  Calciferous  limestones  of  New 
Jersey  which  according  to  Professor  Geo.  H.  Cook  (Geology 
of  New  Jersey  1868)  contain  from  seventeen  to  twenty  per 
cent  of  magnesia. 

In  this  abstract  it  has  not  been  possible  to  give  in  detail  the 
evidence  upon  which  the  writer's  conclusions  are  based.  A 
complete  discussion  of  the  evidence  must  therefore  be  reserved 
for  future  publication. 


